Giphy plugin - fixes & enhancements

[TwilightCoder]

@Alex-D Thank you for the giphy plugin. Here are some quick fixes and enhancements we've performed that seem worth merging upstream. Please let us know what you think.

Fixes:

• Image fallback: Uses fallback image variant when fetching from API search results - images.downsized_medium along with the current images.downsized. images.downsized attribute often seems to be empty, where images.downsized_medium is present [bda5058cfb881ae497bda90c30ca0b419a504c14].
• CSP issue: Moves image onload callback out of inline definition into its own method for CSP compatibility [fb999e98e9cbaac67158689cc240d054c3318de4].

Enhancements:

• Alt tag: Adds alt tag into the images wherever available, from the title attribute in the API result [4ff8b908e2707e26f4eddec779d2a58e89c698dc].

[synth]

@Alex-D Indeed, this fixes 2 major bugs with the Giphy plugin as indicated in the description above. Would love to see this upstream so we can stop.

NOTE: The first issue (image fallback) maps to issue #1243. However, there is no existing issue for the CSP issue (happy to make one if necessary)

[Alex-D]

Thank you :)